181 research outputs found
Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics
A metamorphic event in the Vammala Migmatite Belt (VMB) at ~1.92 Ga, revealed by SHRIMP U-Pb analyses of both zircon overgrowths and monazite, is interpreted as post-depositional and is correlated with the development of the early high-grade schistosity. Neither this Early Svecofennian deformation and metamorphism, nor the associatedcomplex folding, is present in the overlying Tampere Schist Belt (TSB) sequence, consistent with the VMB being part of a pre-1.91 Ga basement complex. The ~1.92 Ga event provides a maximum deposition age for the TSB, confirming earlier age estimates. Earlier stratigraphic correlations between parts of the VMB and TSB, and associated tectonic interpretations, can no longer be sustained. The crustal thickening seen in the VMB, and previously attributed to arc accretion at ~1.89 Ga, is now attributed to accretion of a large Svionian marginal basin during the âEarly Svecofennianâ orogenicphase at ~1.92 Ga. This is of similar age to the deformation and metamorphism associated with collision in the Lapland-Kola Orogen to the north of the Karelian Province. The well-known post-TSB orogenic phase was also identified in the VMB by a monazite age of 1881±6 Ma. A granitoid intrusion gave an emplacement age of 1888±5 Ma,comparable to the age of granitoid clasts in the upper part of the TSB succession. The detrital zircon data are interpreted to suggest that deposition of the precursor VMB sediments probably took place soon after an earlier pre-depositional metamorphism at ~1.98 Ga, which affected igneous source complexes dated at ~1.99 Ga and ~2.01 Ga. Mafic rocks in the southern part of the VMB, and probably also the Haveri basalts, represent a renewed episode of extensional magmatism, which might correlate with the 1.96â1.95 Ga Jormua and Outokumpu ophiolites. A pre-1.96 Ga older stage basin has an expression in Sweden and complexes of similar age occur in theconcealed Palaeoproterozoic basement south of the Gulf of Finland. Similar rocks, deformed and metamorphosed before ~1.96 Ga, might be present beneath the Central Finland granitoid complex and the late Svecofennian granite-migmatite zone, and were possibly more local sources for both the younger stage Svionian basin sediments andthe post-1.91 Ga Bothnian Basin sediments. The TSB and other post-accretionary volcanic sequences, and the associated plutonism,are interpreted to reflect a ~40 m.y. extensional period, inboard of the contemporaneous active margin, between orogenic phases at ~1.92 Ga and ~1.88 Ga. This interpretation provides a more satisfactory explanation of the major heat input to the crust over a very wide area than does the arc accretion hypothesis. The tectonic evolutionof the Svecofennian Province has strong similarities to that of the Palaeozoic Lachlan Fold Belt in eastern Australia
Evaluation of bioluminescence-based assays of anti-malarial drug activity
Transgenic Plasmodium falciparum expressing luciferase offers an attractive bioluminescence-based assay platform for the investigation of the pharmacological properties of anti-malarial drugs. Here a side-by-side comparison of bioluminescence and fluorescence-based assays, utilizing a luciferase reporter cassette that confers a strong temporal pattern of luciferase expression during the S-phase of intraerythrocytic development, is reported
Synthesis of ÎČ-Branched Tryptophan Analogues Using an Engineered Subunit of Tryptophan Synthase
We report that l-threonine may substitute for l-serine in the ÎČ-substitution reaction of an engineered subunit of tryptophan synthase from Pyrococcus furiosus, yielding (2S,3S)-ÎČ-methyltryptophan (ÎČ-MeTrp) in a single step. The trace activity of the wild-type ÎČ-subunit on this substrate was enhanced more than 1000-fold by directed evolution. Structural and spectroscopic data indicate that this increase is correlated with stabilization of the electrophilic aminoacrylate intermediate. The engineered biocatalyst also reacts with a variety of indole analogues and thiophenol for diastereoselective CâC, CâN, and CâS bond-forming reactions. This new activity circumvents the 3-enzyme pathway that produces ÎČ-MeTrp in nature and offers a simple and expandable route to preparing derivatives of this valuable building block
The Local Group Census: planetary nebulae in Sextans B
Five planetary nebulae (PNe) have been discovered in the nearby dwarf
irregular galaxy. Emission line images were obtained using the Wide Field
Camera of the 2.5m Isaac Newton Telescope (INT) at La Palma (Spain). The
candidate PNe were identified by their point-like appearance and relatively
strong [OIII] emission-line fluxes. They are located within a galactocentric
distance of 2.8 arcmin, corresponding to 1.1 kpc at the distance of Sextans B.
Luminosities are in the range 1800--5600Lsolar. Sextans B is one of the
smallest dwarf irregular galaxies with a PN population. The number of PNe
detected suggest an enhanced star formation rate between 1 and 5 Gyr ago.Comment: 7 pages, 2 figure
Identifying Old Tidal Dwarf Irregulars
We examine the observational consequences of the two possible origins for
irregular galaxies: formation from collapse of a primordial cloud of gas early
in the age of the Universe, and formation from tidal tails in an interaction
that could have occured any time in the history of the Universe. Because the
formation from tidal tails could have occurred a long time ago, proximity to
larger galaxies is not sufficient to distinguish tidal dwarfs from traditional
dwarfs. We consider the effects of little or no dark matter on rotation speeds
and the Tully-Fisher relationship, the metallicity-luminosity relationship,
structure, and stellar populations. From these selection criteria, we identify
a small list of dwarf irregular galaxies that are candidates for having formed
as tidal dwarfs.Comment: ApJ, to appear September 20, 200
Tryptophan synthase uses an atypical mechanism to achieve substrate specificity
Tryptophan synthase (TrpS) catalyzes the final steps in the biosynthesis of L-tryptophan from L-serine (Ser) and indole-3-glycerol phosphate (IGP). We report that native TrpS can also catalyze a productive reaction with L-threonine (Thr), leading to (2S,3S)-ÎČ-methyltryptophan. Surprisingly, ÎČ-substitution occurs in vitro with a 3.4-fold higher catalytic efficiency for Ser over Thr using saturating indole, despite >82,000-fold preference for Ser in direct competition using IGP. Structural data identify a novel product binding site and kinetic experiments clarify the atypical mechanism of specificity: Thr binds efficiently but decreases the affinity for indole and disrupts the allosteric signaling that regulates the catalytic cycle
Case report and literature review: transient Inab phenotype and an agglutinating anti-IFC in a patient with a gastrointestinal problem
The Inab phenotype is a rare deficiency of all Cromer antigens. These antigens are carried on the decay-accelerating factor (DAF, CD55) molecule that is attached to the red blood cell (RBC) membrane by a glycosylphosphatidylinositol (GPI) anchor. Although typically inherited, an acquired and transient form of the Inab phenotype also exists. A patient with the triad of transient Inab phenotype, a direct-agglutinating anti-IFC, and gastrointestinal (GI) abnormalities is reported. CASE REPORT: An 18-month-old boy with gastroesophageal reflux disease requiring a feeding tube, milk and soy intolerance, and severe growth retardation, as well as vision and hearing deficits from cytomegalovirus infection, was identified when pretransfusion testing revealed a potent panagglutinin (titer > 2000 at 4°C). This antibody did not react with Dr(aâ) and IFC RBCs, and the autocontrol was negative. The patientâs RBCs lacked CD55 by flow cytometric techniques but had normal levels of CD59 and antigens such as Yt a and Emm, carried on GPI-linked proteins, thus excluding paroxysmal nocturnal hemoglobinuria. Several months after initial detection, the anti-IFC was virtually undetectable and his cells reacted weakly with anti-IFC, anti-Dr a , and anti-CD55. RBCs from the propositusâ parents and brother demonstrated normal CD55 and CD59 expression. CONCLUSION: This is the first example of a direct-agglutinating anti-IFC. The cause of the transient depression in CD55 protein (and thus Cromer system antigens) and appearance of anti-IFC remains unknown, as does the relationship between the patientâs GI system abnormalities and these serologic findings.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71992/1/j.1537-2995.2006.00933.x.pd
Rainfall-Linked Megafires as Innate Fire Regime Elements in Arid Australian Spinifex (Triodia spp.) Grasslands
Large, high-severity wildfires, or "megafires," occur periodically in arid Australian spinifex (Triodia spp.) grasslands after high rainfall periods that trigger fuel accumulation. Proponents of the patch-burn mosaic (PBM) hypothesis suggest that these fires are unprecedented in the modern era and were formerly constrained by Aboriginal patch burning that kept landscape fuel levels low. This assumption deserves scrutiny, as evidence from fire-prone systems globally indicates that weather factors are the primary determinant behind megafire incidence, and that fuel management does not mitigate such fires during periods of climatic extreme. We reviewed explorer's diaries, anthropologist's reports, and remotely sensed data from the Australian Western Desert for evidence of large rainfall-linked fires during the pre-contact period when traditional Aboriginal patch burning was still being practiced. We used only observations that contained empiric estimates of fire sizes. Concurrently, we employed remote rainfall data and the Oceanic Niño Index to relate fire size to likely seasonal conditions at the time the observations were made. Numerous records were found of small fires during periods of average and below-average rainfall conditions, but no evidence of large-scale fires during these times. By contrast, there was strong evidence of large-scale wildfires during a high-rainfall period in the early 1870s, some of which are estimated to have burnt areas up to 700,000 ha. Our literature review also identified several Western Desert Aboriginal mythologies that refer to large-scale conflagrations. As oral traditions sometimes corroborate historic events, these myths may add further evidence that large fires are an inherent feature of spinifex grassland fire regimes. Overall, the results suggest that, contrary to predictions of the PBM hypothesis, traditional Aboriginal burning did not modulate spinifex fire size during periods of extreme-high arid zone rainfall. The mechanism behind this is that plant assemblages in seral spinifex vegetation comprise highly flammable non-spinifex tussock grasses that rapidly accumulate high fuel loads under favorable precipitation conditions. Our finding that fuel management does not prevent megafires under extreme conditions in arid Australia has parallels with the primacy of climatic factors as drivers of megafires in the forests of temperate Australia
Optical Counterparts of Ultra-Luminous X-ray Sources identified from Archival Hubble Space Telescope/WFPC2
We present a systematic analysis of archival HST WFPC2 ``Association'' data
sets that correlate with the Chandra positions of a set of 44 ultra-luminous
X-ray sources (ULXs) of nearby galaxies. We have improved the Chandra-HST
relative astrometry whenever possible. Disparate numbers of potential ULX
counterparts are found, and in some cases none are found. The lack of or low
number of counterparts in some cases may be due to insufficient depth in the
WFPC2 images. Particularly in late-type galaxies, the HST image in the ULX
region was often complex or crowded. We therefore address various scenarios for
the nature of the ULX since it is not known which, if any, of the sources found
are true counterparts. The optical luminosities of the sources are typically in
the range 10^4-6 L_sun. In several cases color information is available, with
the colors roughly tending to be more red in early-type galaxies. This suggests
that, in general, the (potential) counterparts found in early-type galaxies are
likely to be older stellar populations, and are probably globular clusters.
Several early-type galaxy counterparts have blue colors, which may be due to
younger stellar populations in the host galaxies, however these could also be
background sources. In spiral galaxies the sources may also be due to localized
structure in the disks rather than bound stellar systems. Alternatively some of
the counterparts in late-type galaxies may be isolated supergiant stars. The
observed X-ray/optical flux ratio is diluted by the optical emission of the
cluster in cases where the system is an X-ray binary in a cluster, particularly
in the case of a low-mass X-ray binaries in old cluster. (abridged)Comment: 35 pages with 9 figures formatted with emulateapj. Only subset of
figures 1 and 2 are shown, for full version see
http://xassist.pha.jhu.edu/ptak/hst_ulx_pape
Contribution of windfarms to ancillary services
International audienceIn the last dĂ©cade, wind energy has expericnced a substantial growth in Europe with an increase in gĂ©nĂ©ration capaciiy from 2,5 GW in 1995 to 34 GW at (lie end of 2004 This incTease has raised new probĂźems and constraints which led System opĂ©ra tors, electric utilities, governments or regulatory boards to define lechnical requirements for the grid connection of wind farms and more generaily of distributed gĂ©nĂ©ration (DG) units. At flrst, the requirements for wind fanns were rather "soft" (at least softer than for other DG units) and were mainĂźy intended to limit the "disturbances" caused by wind energy on power quality and grid opĂ©ration. But with the ever increasing developmenl of wind power, the impacts on the grids become more and more significant leading to the dĂ©finition of more and more "scvcrc" requiremcnts, In parltcular, wind farms (WF) arc now more and more often askcd to provide some son of anciilary services such as contribution to voltage/rĂ©active power control and rrequency/active power control. This paper focuscs on the possible provision of such anciilary services by wind faims. Regarcling rĂ©active power and voltage contTOl:-Doubly-fed Induction GĂaerarors (DFIG) and Synchronous or Induction GeneratoTS with full power Ă©lectron ics interfaces (SIG) can liave rĂ©active power control capabilities (both in production and absorption) depending on the rating of their power electronics converters. ThĂšse capabilities {characterized by a fast dynamie respon.se) can be used to perform voltage control. Moreover, if required, extemal reactive power compensation device.s niay also be installed.-Classical Induction G encrĂąt ors (OG) do not hĂąve such rĂ©active power control capahilities and therefore require externat devices for reactive power and voltage control.-Simulations carried oui show that WFs with voltage control capabilĂźties can significantly sustaĂźn the network voltage in case of grid events and mus may efficiently support the power System stability.Regard ing frequency control: the resuits show that when the Wind Turbine Generator (WTG) is full y loaded (maximum active power gĂ©nĂ©ration], appre-priate use or the pitch control may enabic the WTG (DFIG, SIG or CIG) to contribute to frequency control. In case of partial toad. the contribution to frequency conlrol can be achieved either by "disoplimizing'" the wind energy conversion by mcans of aie pitch control. or by setting a non-optimal rotor speed for DFIG or SIG. However, a judicious way to use variable speed ge&eratCffS may also be looperate them as mertial flywheels
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